An iron superhero is perhaps the more obvious of the two choices. Iron is a strong metal so the superhero would be impervious to bullets or punches. He could also use his body to smash through brick walls and the like. An oxygen superhero is a bit more interesting. Oxygen is a strong oxidizer and can oxidize many different chemicals. So the oxygen superhero could use his touch (or a "beam") to weaken any metal structures and render them useless. Oxygen is also required for combustion. A burning flame when exposed to oxygen will burn with greater intensity. So he could use his oxygen breath to shoot out fire. Ironically, in a battle between the two, oxygen man would likely defeat iron man since oxygen readily oxidizes iron and turns it into rust.

I think the iron super hero should be able to harness light to make refections and illusions because of iron's luster or the super hero should be able to absorb electricity and shoot it out in a lightning like beam. sorry i don't have any thing for oxygen